(1) Field of the Invention
The present invention generally relates to a cooling system of an internal combustion engine, and more particularly to a cooling system for cooling a cylinder and/or a cylinder head.
(2) Description of the Related Art
Japanese Laid-Open Utility Model Application No. 168242 discloses a cooling system for cooling an internal combustion engine by passing a coolant through a spiral or ring-shaped passage formed on an outer surface of a cylinder liner.
FIGS. 1A, 1B, and 1C are plan, front, and side views of a conventional cylinder liner, respectively. A plurality of ring-shaped passages 11, each formed in-a plane roughly perpendicular to the longitudinal axis of the cylinder liner are spaced apart from each other at equal intervals in a longitudinal axial direction of the cylinder liner 10. All the passages 11 are coupled to each other by means of connection passages 12 and 13 formed in the axial direction of the cylinder liner 10. A coolant is poured into the cylinder liner 10 via an inflow part 14, and is distributed into the ring-shaped passages 11 via the connection passage 12. The cylinder is cooled while the coolant is flowing in the ring-shaped passages 11. After flowing through the cylinder, the coolant flows into the connection passage 13, and the coolant collected in the connection passage 13 is discharged via a flux part 15.
FIGS. 2A and 2B are respectively system diagrams of cooling systems, which are used for multi-cylinder engines; these cooling systems cool cylinders by means of such cylinder liners.
In the system shown in FIG. 2A, coolant is pressurized by a pump 21, and then the coolant flows in parallel cylinder coolant passages 23a, 23b, 23c, 23d. An outlet of the pump 21 separates into the passages 23a-23d each of which passages is provided for the respective cylinders, which cylinders are formed in a cylinder block 22. After flowing through the passages 23a-23d, the coolant is collected and then it flows into a cylinder head coolant passage 25. The coolant thus returns to the pump 21 via a radiator 26 and a thermostat 27.
In a system shown in FIG. 2B, coolant pressurized by a pump 21 is distributed into a cylinder block 22 side and a cylinder head 24 side, then coolant distributed into the cylinder block 22 side flows into cylinder coolant passages 22a, 22b, 22c and 22d, while coolant distributed into the cylinder head 24 side flows into a cylinder head coolant passage 25, in parallel. Both of the coolant flows are thus collected so as to flow into a radiator 26.
The cylinder coolant passages such as those shown in FIGS. 1A through 1C have a high cooling ability. In a construction of a cooling system such that cold coolant is directly provided to the cylinder coolant passages, in a cold state of the engine, a temperature on an inner surface of a cylinder liner is too low, and friction energy loss due to a piston reciprocating in the cylinder liner will increase. Further, the clearance between the outer surface of the cylinder liner and the inner surface of a bore of the cylinder block will increase as the temperature of the liner decreases. As the cylinder liner is fitted in the bore, this results in a diminishing of the sealing ability for sealing coolant in a space between the outer surface of the cylinder liner and the inner surface of a bore of the cylinder block.
Further, in the construction shown in FIGS. 2A and 2B, bubbles generated in the pump 21 flow into the cylinder coolant passages 24a through 24d directly. It is then difficult to remove the bubbles in the cylinder coolant passages 24a-24d, which are constructed as shown in FIGS. 1A-1C. There has been a problem in that these bubbles staying in the passages 24a-24d may make the cylinder liner corrode.
A cooling system, wherein coolant is provided to coolant passages of a cylinder liner after passing through a cylinder head coolant passage, is disclosed in Japanese Laid-Open Patent Application No. 2-130246. However, this cooling system may cause a problem such that a sufficient amount of coolant may not be provided to cylinder coolant passages if the cooling system is applied to an engine having a plurality of cylinders, because pressure of the coolant may be expended during passage through a cylinder head cooling passage before the coolant flows into the cylinder coolant passages. Thus, this lack of coolant pressure, and resulting lack of coolant supplied to the cylinder coolant passages prevents the cylinders from being cooled sufficiently.
A combustion chamber of an engine consists of a cylinder block and a cylinder head, the cylinder block having some cylinders located therein, and the cylinder head having a concavity on a bottom surface thereof. Outer surfaces of cylinder liners are fitted in inner surfaces of bore parts formed in the cylinder block. Thus, a high temperature generated by engine operation in the combustion chamber result in heat being transmitted into the cylinder liners and other parts via the cylinders and the cylinder head.
The present applicant disclosed a cooling system having a construction such as shown in FIG. 3 in Japanese Patent Application No. 3-51701. A cooling system such as the above mentioned is a cooling system of an internal combustion engine, the cooling system effecting cooling by means of the so-called groove cooling method. The groove cooling method is a method wherein coolant flows into coolant passages. The coolant passages are formed between the inner surfaces of the bore parts of the cylinder block and the outer surfaces of the cylinder liners. The coolant flowing into the coolant passages cools a wall of the cylinder liner and the flowing of coolant in the coolant passages prevents the coolant from boiling.
In the FIG. 3, a plurality of ring-shaped grooves 33 each having rectangular sectional areas are formed on an outer surface of a cylinder liner 32 each in a plane perpendicular to a longitudinal axial direction of the cylinder liner 32. The plurality of ring-shaped grooves 33 are spaced apart from each other at equal intervals along a longitudinal axis of the cylinder liner. The cylinder liner 32 is fitted in a cylinder block 31. Ring-shaped coolant passages are formed by the plurality of ring-shaped grooves 33 when each cylinder liner 32 is fitted into corresponding bore parts of the cylinder block 31.
Further, longitudinal grooves 35a, 35b, 36a, and 36b are formed on both the outer surface of each cylinder liner 32 and the inner surface of the corresponding bore part of the cylinder block 31, along the longitudinal axial direction of the cylinder liner 32, in positions such that corresponding longitudinal grooves are opposite to each other, so as to form passages connecting the above mentioned ring-shaped grooves 33 with each other. Also inflow passages 37a, 37b are formed in the cylinder block 31. One side of each of the passages is connected with the respective longitudinal grooves 35a and 35b. Flux passages 38a and 38b are formed in the cylinder block 31, in a diametrically opposite location. One side of each of the passages is connected with the respective longitudinal grooves 36a and 36b.
A pump 39 discharges coolant so as to distribute it into two flow paths of coolant. The pump 39 then provides one of the coolant flow paths to the inflow passage 37a via a filter 40, at a high pressure thereof, while the pump 39 provides the other coolant flow path to the flux passage 37b directly, at a low pressure thereof. The coolant in the inflow passage 37a is distributed into the ring-shaped grooves located in an upper side, as in FIG. 3, of the cylinder liner 32 via the longitudinal groove 35a, then the coolant passes around the outer surface of the cylinder liner 32 and is consequently discharged from the longitudinal groove 36a via the flux passage 38a. On the other hand, the coolant in the inflow passage 37b is distributed into the ring-shaped grooves located in a lower side, as in FIG. 3, of the cylinder liner 32 via the longitudinal groove 35b, then the coolant passes around the outer surface of the cylinder liner 32, and is consequently discharged from the longitudinal groove 36b via the flux passage 38b. The coolant discharged from the flux passages 38a and 38b is collected so as to be returned to the pump 39 via a radiator (not shown in the drawing).
In the above mentioned disclosed system, heat generated in the combustion chamber is transmitted into the cylinder liner via the cylinder head, the heat is reduced by means of cooling the wall of the cylinder liner 32. An incoming heat distribution of the wall of the cylinder liner 32 is such that an upper part, as in FIG. 3, which part is nearest to the combustion chamber, is the hottest part and the lower part is at a lower temperature.
Thus, to make it possible to cool the wall of the cylinder liner 32 uniformly, it is necessary that a coolant flow rate in the ring-shaped grooves 33 located nearest to the combustion chamber is largest, and that a coolant flow rate in the ring-shaped grooves located farther from the combustion chamber, is smaller, as shown by a solid line curve `c` of FIG. 4.
In the system shown in FIG. 3 if sectional areas of the longitudinal grooves 35a, 35b, 36a, 36b are larger than a predetermined amount, a distribution of coolant flow rate in the ring-shaped grooves 33 becomes such that a flow rate is uniform in all parts of each of the ring-shaped grooves 33 between the uppermost part and the lowest part thereof as shown by a broken line `a` of FIG. 3. On the other hand, if the sectional areas of the longitudinal grooves 35a, 35b, 36a, and 36b are reduced to an amount less than the predetermined amount, a flow rate in an upper part of the ring-shaped grooves 33, as in FIG. 3, is greater as shown by a broken line `b` of FIG. 4, so that an incoming heat distribution in the ring-shaped grooves 33 becomes more similar to that shown in the solid line `c` of FIG. 4.
However, it may be difficult to equate a distribution of flow rate of coolant flowing in the ring-shaped grooves to the distribution of the heat incoming to the cylinder liner 32 because the difference of flow rates is too large. The difference of flow rates occurs between a flow rate in an uppermost part of the ring-shaped grooves and a flow rate in a lower part of the ring shaped grooves. This difference is shown by the broken line curve `b` of FIG. 4. The reason for the above mentioned difference in produced flow rates will be described below. The inflow passage 37a and the longitudinal groove 35a are connected with each other so as to intersect each other at a right angle at a connecting point thereof. Coolant flowing into the uppermost groove 33 flows straight and does not have to make the right angle turn at the connecting point between the inflow passage 37a and longitudinal groove 35a. The groove 33 extends along the same line as a line in which the inflow passage 37a extends, as shown in FIG. 3. On the other hand, coolant flowing into the ring-shaped grooves 33 located lower than uppermost groove 33, must make the right angle turn at the connecting point between the inflow passage 37a and the longitudinal groove 35a. As a result, a flow rate of coolant flowing in the ring-shaped grooves 33 located lower than the uppermost groove 33 is small, because of the large pressure loss incurred in the right-angle connecting point between the inflow passage 37a and the longitudinal groove 35a. Therefore, coolant flows mainly along arrows shown in the FIG. 3.
Another example of a cooling system having ring-shaped passages formed in a cylinder liner in a fitted position in the cylinder block relating to the present invention is shown in FIG. 5. In FIG. 5, a groove 4 having a rectangular sectional area is formed on an outer surface 53 of the cylinder liner 51 spirally, the cylinder liner 51 having a rim part 52 at a top position thereof. The cylinder liner 51 is fitted into an inner surface 56 of a bore formed in a cylinder block 55 so that the outer surface 53 of the cylinder liner 51 is in contact with the inner surface 56 of the bore of the cylinder block 55. Thus, a spiral-shaped coolant passage 57 consists of the groove 54 and the inner surface 56 of the bore.
In this construction of the coolant passage of the cooling system, a shock absorbing function of the cylinder liner 51 is not sufficient when an inner wall of the cylinder liner 51 is deformed by a side pressure of a piston or a combustion pressure, due to contact of the outer surface 53 of the cylinder liner 51 with the inner surface 56 of the bore of the cylinder block 55. Thus, the inner wall of the cylinder liner 51, which is constructed to retain engine oil thereon, suffers from mirror abrasion due to a deficiency of the shock absorbing function of the cylinder liner 51. Therefore, there have been problems with the retaining-oil function of the inner wall of the cylinder liner 51 becomes diminished, and noise generated by piston movement becomes large.
Further, there has been another problem with electrolyte etching caused in the cylinder liner surface 51 which is in contact with the bore of the cylinder block 55. Electrolyte etching occurs if the cylinder liner 51 is made of cast iron and the cylinder block 55 is made of aluminium. As electrolyte etching is caused when metals of different kinds are in contact with each other.